The present invention relates generally to firing circuits for thyristor power conversion systems and more particularly to an improvement for controlling the commutation of a load commutated inverter which supplies a synchronous machine from a polyphase alternating current source.
Many circuits and systems are known for controlling the conductivity of controlled rectifiers utilized in various types of converters for supplying electrical power to a load such as an AC motor, from a polyphase alternating current (AC) source. The type of rectifier used will, of course, control to some degree the type of control utilized, but by far the most common control rectifier used today is a thyristor of the silicon controlled rectifier type which becomes conductive with the simultaneous application of a forward bias voltage and a signal applied to its gate electrode and which thereafter remains conductive until the anode current falls below the value required to hold the thyristor in the conductive state.
With respect to the load commutated inverter for a three phase (3.phi.) AC motor drive, control of the inverter supplying the synchronous motor is normally based upon a thyristor firing strategy of firing the thyristor as late as possible. To "fire as late as possible" is to render the thyristors conductive at the commutation limit point; i.e., at a power factor angle just sufficiently leading to provide the volt-seconds necessary to safely commutate the current from one thyristor to the other. It is to this aspect that the present invention is directed.
It should also be pointed out that whereas motor control systems employing thyristors have been implemented using analog control techniques, a typical example being U.S. No. 4,230,979, entitled, "Control Current Inverter And Motor Control System," Paul M. Espelage, et al., which issued on Oct. 28, 1980, attention has been and is presently being directed to digital type of control techniques, examples of which are disclosed in: U.S. Pat. No. 3,601,674, "Control System For Firing SCR's In Power Conversion Apparatus", John A. Joslyn, et al., which issued on Aug. 24, 1971; U.S. Pat. No. 4,263,557, "Power Converter Control", Willard B. Jarvinen, which issued on Apr. 21, 1981; and U.S. Pat. No. 4,276,505, "Microcomputer-Based Control Apparatus For a Load-Commutated Inverter Synchronous Machine Drive System", Bimal K. Bose, which issued on June 30, 1981. The teachings of these patents are also intended to be incorporated herein by reference.
In any phase control system, whether it be an analog or digital type system, the AC terminal voltage is a prime feedback signal employed for the thyristor bridge control. Typically, the AC terminal or phase voltages, referred to a fictitious neutral, are coupled from the high voltage thyristor bridge through a high impedance resistor attenuator string into differential amplifiers in the control circuitry and to other related circuitry to further derive various signals for a variety of purposes such as thyristor state detectors and voltage regulation. The principal use of the terminal voltage, however, is in a phase lock loop firing control circuit wherein synchronizing signals are generated from processed line voltages, typically involving integration of the AC line to line voltages. In such applications, the zero voltage commutation notches appearing in the line-to-line voltages generate flat spots in the integrated output voltage with the positioning of the notches being dependent upon the actual firing angle and the duration of the notches being dependent upon the line current and the inductive reactance in the commutation path. Typically, the zero crossings of the integrated line to line voltages are determined by comparators which are utilized to form a synchronizing pulse train at six times line frequency. Since the aforementioned flat spots can occur at the zero crossings, the stability of the phase lock loop can be undesirably affected. To overcome this deleterious effect, the above cross referenced application, U.S. Ser. No. 300,200 entitled, "Line to Line Voltage Reconstruction For Synchronizing Thyristor Power Converter", discloses an improved technique for removing the commutation notches by summing the integrated output of the corrupted line to line voltage with a signal proportional to the commutating inductance multiplied by the fictitious "delta" current which is derived by taking the difference between the actual line currents in a manner well known to those skilled in the art. The resultant or composite voltages are the primary feedback control signals for synchronizing either a fixed frequency source side converter or a variable frequency load side converter or both.